1. Field of the Invention
The present invention relates to laminated thermoplastic films. In particular, oriented polypropylene films are provided with a relatively thin surface coating of heat-sealable polymer material. The laminated oriented polypropylene film exhibits good heat-seal strength, broad heat-seal temperature range, and excellent optical properties.
2. Description of the Prior Art
Biaxially oriented polypropylene flms have been widely used in the past as packaging films, especially in the area of food packaging, since such films offer excellent optical, mechanical, and barrier properties. Expanded usage of such films, however, has been limited by the narrow heat-seal range thereof. Such films, moreover, at the elevated temperature requisite for heat sealing, characteristically pucker and tear. A means used in the past to overcome this and to allow such films to be satisfactorily sealed over a wide temperature range has been to apply heat-sealable coatings to the surface of the film. For example, the polypropylene has been coextruded with surface layers of an ethylene-propylene random copolymer and subsequently oriented. Although such films exhibit high seal strength, because of the relatively high melting point of the copolymer skin, the temperature range over which usable seals are formed is narrow. Similarly, oriented coextruded film has been prepared in which the skins are comprised of low density polyethylene which has been polymerized in a free radical-catalyzed process. In such case, because the melting point of the polyethylene skin is lower than that of the ethylene-propylene copolymer, a broader sealing range can be achieved. However, such films have shown poor adhesion between the skin and core layers showing an increased tendency to delaminate in heat-sealing operations. Consequently, the seal strengths provided by skins comprised of polyethylene which has been polymerized by a free radical-catalyzed process are not as high as those provided by an ethylene-propylene random copolymer skin. Moreover, the haze and gloss values of such films are poorer than those exhibited by uncoated oriented polypropylene films.
Additional problems encountered when attempting to form a coextruded biaxially oriented polyethylene coated polypropylene film include the tendency of the lower melting point polyethylene to adhere to the surface of draw rolls commonly employed to achieve machine direction orientation of the laminate. Attempts to remedy this problem are disclosed in U.S. Pat. No. 4,147,827, the disclosure of which is incorporated herein by reference. In that patent, the core polypropylene material is blended with another resin which has a plasticizing effect upon the modified polypropylene. This allows the coextruded base web, carrying the polyethylene coating, to be stretched at temperatures below which adhesion of the coating to the draw rollers occurs. Machine direction stretch at such low temperatures, however, may, under particular operating conditions, result in fibrillation of the modified polypropylene core material whereby the resultant biaxially oriented laminate may exhibit impaired physical properties. The following Table 1 sets forth specific physical property improvements when an unmodified polypropylene base web is compared to a polypropylene core layer which has been modified by the addition of a plasticizing agent, described by the manufacturer as an interpolymer of alpha-methylstyrene; aliphatic C.sub.5 olefins; and terpenes.
TABLE 1 ______________________________________ Property Plain Core Modified Core ______________________________________ *WVTR 0.35 0.45 Ball Burst Strength 15 1 (ASTM-905-37) MO Ultimate (Psi) 23,000 2,000 Shrinkage at 275.degree. F. 5 to 7% 10 to 20% ______________________________________ *Water Vapor Transmission Ratio
It can be seen from the foregoing Table that attempts to modify the polypropylene base layer to effect machine direction orientation at low temperatures, result in a base layer having markedly reduced physical strength characteristics as well as lower barrier properties and undesirable shrinkage characteristics.